The present invention relates to a method for providing heat cured, precious metal catalyzed, solventless silicone release coating compositions which have improved shelf-life and bath-life, require utilization of smaller quantities of precious metal catalyst and provide the end-user greater flexibility in formulating and preparing release coating baths which meet his specific needs. In addition, the present invention provides an article of manufacture useful for providing the ingredients necessary to practice the foregoing method.
An important advance in the art of silicone release coating compositions is the solventless release coating composition of Grenoble, described in U.S. Pat. No. 4,071,644. This composition includes a low viscosity vinyl chain-stopped polysiloxane or a low viscosity silanol terminated polysiloxane with an organohydrogensiloxane and a curing catalyst. Among the advantages of such a solventless release coating composition are that the need for expensive pollution control equipment is eliminated and the amount of energy required to effect curing is substantially reduced.
However, a problem relating to premature curing or gelation must be overcome in order to provide a commercially more desirable solventless release coating composition. A typical coating composition is provided as a two component system, one component containing a base polymer and catalyst and the other a cross-linking agent. When the two components are mixed together to provide a release coating bath, premature curing may result in a catalyzed bath having a pot-life insufficient to enable the end-user to carry out the coating application. One way of ameliorating these problems is to include an inhibitor in the component containing the base polymer and catalyst which is effective for retarding the cure reaction at ambient temperature but not at elevated temperatures.
One example of such an inhibitor containing release coating composition is disclosed in Eckberg, U.S. Pat. No. 4,256,870. This release coating composition comprises a base polymer such as a vinyl chain-stopped polydialkyl-alkylvinyl copolymer and a methylhydrogen cross-linking fluid. A platinum or platinum metal complex is provided to initiate an addition cure or hydrosilation reaction. An inhibitor selected from organic esters of maleic acid is added to the composition in order to selectively retard the cure reaction so as to provide adequate shelf-life and a workable pot-life for the coating composition.
Eckberg in U.S. Pat. No. 4,262,107 discloses a silicone rubber composition with a good shelf-life and fast curing at elevated temperatures comprising a silanol end-stopped diorganopolysiloxane polymer, a silicone hydride and a rhodium complex catalyst in combination with an inhibitor compound selected from low molecular weight silanol end-stopped diorganopolysiloxanes, certain acetylenic compounds, olefinic carboxylic acid esters of aliphatic alcohols such as vinyl acetate, alkenyl isocyanurates and mixtures of such inhibitor compounds.
In U.S. Pat. No. 4,347,346 Eckberg discloses a solventless platinum or rhodium catalyzed addition curable polyorganosiloxane release coating composition containing a dialkylacetylenedicarboxylate inhibitor for preventing premature gelation at ambient temperatures but which will cure at elevated temperatures via a hydrosilation reaction to form a coating capable of releasing materials from substrates which would normally adhere thereto.
A solventless silicone release coating composition containing a blend of a dialkylcarboxylic ester having carbon to carbon unsaturation and an olefinic carboxylic acid ester of an aliphatic alcohol which is effective for retarding a precious metal catalyzed addition cure hydrosilation reaction at room temperature, but which does not interfere with curing at elevated temperatures, is provided by Eckberg in Ser. No. 458,420, filed concurrently herewith. Generally such release coating composition comprises an olefinorganopolysiloxane or silanol terminated polysiloxane base polymer, an organohydrogenpolysiloxane cross-linking agent, a catalytic amount of a precious metal catalyst and an effective amount of a blend of a dialkylcarboxylic ester having carbon to carbon unsaturation and an olefinic carboxylic acid ester of an aliphatic alcohol for inhibiting an addition cure hydrosilation reaction between said base polymer and said cross-linking agent at room temperature, but which is not present in an amount sufficient to prevent curing of said coating composition at elevated temperatures. The ratio of dialkyl carboxylic ester to olefinic carboxylic acid ester of an aliphatic alcohol of the inhibitor blend ranges from approximately 1:24 to 1:1, but preferably is blended in a 1:3 ratio. The most preferred dialkyl carboxylic ester containing carbon to carbon unsaturation is diallylmaleate; however, other preferred compounds include butylallylmaleate, diallylsuccinate, dimethylmaleate, diethylmaleate and silylmaleates such as bis.gamma.-propyltris (methoxy) silylmaleate. The most preferred olefinic carboxylic acid ester of an aliphatic alcohol is vinyl acetate; however, any compound derived from an aliphatic alcohol having 1 to 4 carbon atoms and an olefinic carboxylic acid having 2 to 10 carbon atoms is also effective. Ordinarily an effective amount of inhibitor ranges from approximately 0.05% to approximately 2% by weight and preferably from 0.1% to 1.5% by weight.
Although compositions such as the foregoing have improved shelf-life and pot-life, it is desirable to provide compositions and methods which still further improve such compositions' shelf-life and pot-life. Additionally, it is desirable to provide compositions and methods which utilize even smaller amounts of precious metal catalyst, further reduce the amount of energy required to effect a cure, and provide the end-user with greater flexibility in formulating and preparing release coating compositions which meet his specific needs. One such composition is provided by Eckberg in U.S. Pat. No. 4,340,647, which comprises a vinyl or silanol functional base polymer, a cross-linking agent containing silanic hydrogen, a precious metal catalyst such as platinum or rhodium, a carboxylic ester inhibitor, and a silicone vinyl gum cure accelerator effective for allowing thermal curing.
All of the foregoing disclosures are assigned to the same assignee as the present invention and are incorporated by reference into the instant disclosure.
Normal procedure for providing the end-user with a solventless release coating composition is to package the base polymer, catalyst and inhibitor in one package and to provide a second package containing the methylhydrogen cross-linking agent. The end-user produces a release coating composition bath by mixing the two packages according to specified proportions, whereupon such mixture is applied to a substrate such as paper and allowed to cure at elevated temperatures in a relatively short period of time.
Fabricators and formulators in the silicone industry have quickly recognized the significance of the foregoing silicone release coating compositions. It has become an industry standard to utilize such compositions in a two-package format at about a 100:5 mixture ratio. This system works quite well as evidenced by the success of the products in the market, but possesses certain disadvantages which are substantially overcome by the present invention.
The present invention provides a system for packaging and formulating silicone release coating compositions and an article of manufacture useful in such system. As a result, the shelf-life and pot-life are increased, energy and precious metal catalyst consumption are reduced, and flexibility in formulating baths to meet specific needs is provided. The two package prior art system effectively fixes the ratio of catalyst and inhibitor and thereby limits the end-user's flexibility. Not only does the three or more package system of the present invention provide such flexibility through the selective use of catalyst and inhibitor, it does so at effective catalyst levels which are lower and therefore more economical than those previously provided by the art.
A system simlar to that of the present invention is described by Caprino in U.S. Pat. No. 4,322,320. Caprino provides a process for formulating an addition cured silicone rubber composition comprising mixing 95 to 99.5 parts of Component A with 0.5 to 5.0 parts of Component B and 0.5 to 5.0 parts of Component C and curing said mixture. Component A is a silicone base compound consisting essentially of 100 parts by weight of a vinyl-containing linear polysiloxane and 0.5 to 50.0 parts by weight of a hydrogen-containing polysiloxane. Component B is a cure agent consisting essentially of 100 parts of said vinyl-containing polysiloxane containing therein at least 0.40 parts of an inhibitor compound having at least one radical of the formula --C--O--O--H, and at least 0.001 parts platinum per 100 parts vinyl polymer in Component B. Component C is a cure modifying agent consisting essentially of 100 parts of said vinyl-containing polysiloxane containing at least 0.10 parts of said inhibitor compound.
The present inventors are the first to recognize that by keeping each ingredient in a separate package improved results are obtained. That is, only the present inventors have recognized the need to keep the catalyst separate from the inhibitor and the base polymer, as well as the need to keep separate the base polymer and cross-linking agent in order to provide a release coating having improved shelf-life, extended workable pot-life and which utilizes substantially less precious metal catalyst than required in the prior art.
It is therefore an object of the present invention to provide a method for increasing the shelf-life and workable pot-life of a silicone release coating composition.
It is also an object of the present invention to provide a method for reducing the amount of precious metal catalyst utilized to initiate curing of a silicone release coating composition without sacrificing performance.
Another object of the present invention is to provide a method for enabling an end-user of a silicone release coating composition to more easily formulate and prepare a composition specific to his needs.
Still another object of the present invention is to provide an article of manufacture comprising a plurality of components which are mixed together to form a silicone release coating composition.
These and other objects will become apparent to those skilled in the art upon careful consideration of the following specification, examples and claims.